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A flow-foliated ignimbrite related to the Åland rapakivi granite in SW Finland |
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| Authors: | Olav Eklund Alexey Shebanov Soren Fröjdö Kari Yli-kyyny Ulf B Andersson |
| Institution: | Department of Geology, Turku University FIN-20014 Turku Finland;Department of Mineralogy, Geological Faculty of St. Petersburg University, 719 University Embtnkment RUS-199034 St.Peterburg, Russia;Department of Geology and Mineralogy, Åbo Akademi University, FIN-20500 Turku /Åbo, Finland;Institute of Earth Sciences, Uppsaln University S-75236, Uppsala, Sweden |
| Abstract: | A flow-foliated felsic ignimbrite constitutes the uppermost lithological unit of the 1.58 Gyr anorogenic magmatic rocks in SW Finland. The ignimbrite is derived from an explosive eruption of hot (≅ 950 °C) phenocryst-bearing A-type (rapakivi-type granite magma. The ignimbrite is close in composition to subvolcanic rapakivi granites that occur in the margins of the kand rapakivi batholith. The subvolcanic granites crystallized under a pressure of ≅ 1 kbar and at temperatures of about 650–700 °C. However, both major and rare earth elements show that the ignimbrite- forming magma was more fractionated than the magma forming the subvolcanic varieties. Supported by evidence of mafic-felsic magma mingling, it is suggested that injection of hot mafic magma into a shallow magma chamber produced the high temperature of the ignimbrite-forming magma. This injection increased the magmatic and the volatile pressure that caused the eruption of the dry felsic magma. |
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